Anode of lithium battery and lithium battery using the same

ABSTRACT

An anode of a lithium battery includes a composite film, the composite film includes a carbon nanotube film structure and a plurality of nanoscale tin oxide particles dispersed therein. A lithium battery includes at least a cathode, an electrolyte, and the anode mentioned above. A charge/discharge capacity of the lithium battery using the anode can be improved.

RELATED APPLICATIONS

This application is a continuation of U.S. Patent Application, entitled“ANODE OF A LITHIUM BATTERY AND METHOD FOR FABRICATING THE SAME” withapplication Ser. No. 12/080,714, filed on Apr. 4, 2008 which claims allbenefits accruing under 35 U.S.C. §119 from China Patent Application No.200710077110.8, filed on Sep. 14, 2007, in the China IntellectualProperty Office, the contents of which are hereby incorporated byreference. This application is related to commonly-assigned applicationentitled, “CATHODE OF LITHIUM BATTERY AND METHOD FOR FABRICATING THESAME”, with application Ser. No. 12/080,717, filed on Apr. 4, 2008.Disclosure of the above-identified application is incorporated herein byreference.

BACKGROUND

1. Technical Field

The present disclosure relates to anodes of lithium batteries andlithium batteries using the same, particularly, to acarbon-nanotube-based anode of a lithium battery and a battery using thesame.

2. Discussion of Related Art

In recent years, lithium batteries have received a great amount ofattention. Lithium batteries are used in various portable devices, suchas notebook PCs, mobile phones and digital cameras. Generally, lithiumbatteries have small weight, high discharge voltage, long cyclic lifeand high energy density compared with conventional lead storagebatteries, nickel-cadmium batteries, nickel-hydrogen batteries, andnickel-zinc batteries.

An anode of a lithium battery should have such properties as high energydensity; low open-circuit voltage versus metallic lithium electrode;high capacity retention; good performance in common electrolytes; highdensity (e.g. >2.0 g/cm³); good stability during charge and dischargeprocesses and low cost. At present, the most widely used anode activematerial is carbonous/carbonaceous material such as natural graphite,artificial graphite and amorphous-based carbon. Amorphous-based carbonhas excellent capacity, but the irreversibility thereof is relativelyhigh. The theoretical maximum capacity of natural graphite is 372 mAh/g,but the lifetime thereof is generally short.

To be used as an anode active material, compared with thecarbonous/carbonaceous materials, a metallic material with high capacityis used. For example, the capacity of tin can be above 1000 mAh/g. Thus,in most lithium batteries, a tin film is formed on a current collectorto achieve the anode. However, during charge/discharge processes, thesize of the anode can increase by as much as 600%. As such, the tin filmmay be pulverized. As such, the capacity of the lithium battery usingtin anode decreases rapidly due to the above-described processes.

What is needed, therefore, is to provide an anode of a lithium batteryand a lithium battery using the same in which the above problems areeliminated or at least alleviated.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present carbon-nanotube-based anode of a lithiumbattery and the related method for fabricating the same can be betterunderstood with reference to the following drawings. The components inthe drawings are not necessarily to scale, the emphasis instead beingplaced upon clearly illustrating the principles of the presentcarbon-nanotube-based anode of a lithium battery and the related methodfor fabricating the same.

FIG. 1 is a schematic view of an anode of a lithium battery, inaccordance with a present embodiment.

FIG. 2 is a schematic view of a composite film used in the anode of thelithium battery of FIG. 1.

FIG. 3 is a flow chart of a method for fabricating the anode of thelithium battery of FIG. 1.

FIG. 4 shows a Scanning Electron Microscope (SEM) image of a carbonnanotube film structure, in accordance with the present embodiment.

FIG. 5 shows a Transmission Electron Microscope (TEM) image of thecomposite film of FIG. 2.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate at least one preferred embodiment of the presentcarbon-nanotube-based anode of lithium battery and the related methodfor fabricating the same, in at least one form, and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION

Reference will now be made to the drawings to describe, in detail,embodiments of the present carbon-nanotube-based anode of lithiumbattery and related method for fabricating the same.

Referring to FIG. 1 and FIG. 2, an anode 10 in the present embodimentincludes a current collector 12 and a composite film 14 disposed on thecurrent collector 12. The supporting member 12 can, beneficially, be ametal substrate. Quite suitably, the metal substrate is a copper sheet.The composite film 14 includes a carbon nanotube film structure 16 and aplurality of nanoscale tin oxide particles 18 dispersed in the carbonnanotube film structure 16.

The carbon nanotube film structure 16 includes at least two overlappedcarbon nanotube films. Each carbon nanotube film includes a plurality ofsuccessive carbon nanotube bundles joined end to end and are aligned inthe same direction. The at least two carbon nanotube films cross andoverlap with each other. The number of the carbon nanotube films and theangle between the aligned directions of two adjacent carbon nanotubefilms is arbitrarily set.

In the present embodiment, a width of the carbon nanotube film structure16 can, suitably, be in the approximate range from 1 centimeter to 10centimeters, and a thickness of the carbon nanotube film structure 16can, usefully, be in the approximate range from 0.01 micron to 100microns. The carbon nanotube film structure 16 having a plurality ofmicropores defined by the spacing/gaps between adjacent carbon nanotubebundles. The diameter of the resulting micropores can, beneficially, beless than 100 nanometers.

Referring to FIG. 4, quite suitably, the carbon nanotube film structureincludes 200 carbon nanotube films overlapping each other. In thepresent embodiment, the width of the carbon nanotube film structure 16is about 5 centimeters, and the thickness of the carbon nanotube filmstructure 16 is about 50 microns. The angle between the aligneddirections of two adjacent carbon nanotube films is about 90°. Thediameter of each of the micropores is about 60 nanometers.

The nanoscale tin oxide particles 18 are adsorbed to the wall of thecarbon nanotubes by van der Waals attractive force or filled into thespacing between adjacent carbon nanotube bundles of the carbon nanotubefilm structure 16. Quite suitably, the diameter of the nanoscale tinoxide particles 18 is in the approximate range from 3 nanometers to 10nanometers. In the present embodiment, the diameter of the nanoscale tinoxide particles 18 is about 6 nanometers.

It is to be understood that, the current collector 12 in the anode 10 ofthe lithium battery in the present embodiment is optional. In anotherembodiment, the anode 10 of the lithium battery may only include thecarbon nanotube film structure 16. Due to a plurality of carbon nanotubefilms being overlapped to form a self-sustained and stable filmstructure, the carbon nanotube film structure 16 can be used as theanode 10 in the lithium battery without the current collector 12.

The carbon nanotube film structure 16 in the present embodiment hasextremely large specific area. As such, a relatively large amount ofnanoscale tin oxide particles 18 can be adsorbed to the walls of thecarbon nanotubes or filled into the micropores of the carbon nanotubefilm structure 16. Accordingly, the lithium battery using theabove-described anode 10 has a large charge/discharge capacity owing tothe nanoscale tin oxide particles 18, and a low discharge voltage owingto the carbon nanotubes. Further, because the carbon nanotube filmstructure 16 is stable, the volume change of the tin is restrictedduring the charge/discharge cycles.

Referring to FIG. 3, a method for fabricating the anode 10 of thelithium battery includes the steps of: (a) providing an array of carbonnanotubes, quite suitably, providing a super-aligned array of carbonnanotubes; (b) pulling out at least two carbon nanotube films from thearray of carbon nanotubes, by using a tool (e.g., adhesive tape oranother tool allowing multiple carbon nanotubes to be gripped and pulledsimultaneously) to form a carbon nanotube film structure 16; and (c)dispersing a plurality of nanoscale tin oxide particles 18 in the carbonnanotube film structure 16 to form a composite film 14; (d) providing acurrent collector 12 and disposing the composite film 14 on the currentcollector 12 to achieve the cathode 10 of the lithium battery.

In step (a), a given super-aligned array of carbon nanotubes can beformed by the steps of: (a1) providing a substantially flat and smoothsubstrate; (a2) forming a catalyst layer on the substrate; (a3)annealing the substrate with the catalyst at a temperature in theapproximate range of 700° C. to 900° C. in air for about 30 to 90minutes; (a4) heating the substrate with the catalyst at a temperaturein the approximate range from 500° C. to 740° C. in a furnace with aprotective gas therein; and (a5) supplying a carbon source gas into thefurnace for about 5 to 30 minutes and growing a super-aligned array ofthe carbon nanotubes from the substrate.

In step (a1), the substrate can, beneficially, be a P-type siliconwafer, an N-type silicon wafer, or a silicon wafer with a film ofsilicon dioxide thereon. Quite suitably, a 4-inch P-type silicon waferis used as the substrate.

In step (a2), the catalyst can, advantageously, be made of iron (Fe),cobalt (Co), nickel (Ni), or any combination alloy thereof.

In step (a4), the protective gas can, beneficially, be made up of atleast one of nitrogen (N₂), ammonia (NH₃), and a noble gas. In step(a5), the carbon source gas can, advantageously, be a hydrocarbon gas,such as ethylene (C₂H₄), methane (CH₄), acetylene (C₂H₂), ethane (C₂H₆),or any combination thereof.

The super-aligned array of carbon nanotubes can, opportunely, be in aheight of about 200 to 400 microns and includes a plurality of carbonnanotubes parallel to each other and approximately perpendicular to thesubstrate. The super-aligned array of carbon nanotubes formed under theabove conditions is essentially free of impurities, such as carbonaceousor residual catalyst particles. The carbon nanotubes in thesuper-aligned array are packed together closely by van der Waalsattractive force. The carbon nanotubes in the super-aligned array ofcarbon nanotubes can, beneficially, be selected from single-walledcarbon nanotubes, double-walled carbon nanotubes, multi-walled carbonnanotubes, or any combination thereof.

It is to be understood that the method for providing the array of carbonnanotubes is not restricted by the above-mentioned steps, but any method(e.g. a laser vaporization method, or an arc discharge method) known inthe art.

In step (b), carbon nanotube films can, beneficially, be pulled out fromthe super-aligned array of carbon nanotubes by the substeps of: (b1)selecting a plurality of carbon nanotube segments having a predeterminedwidth; (b2) pulling the carbon nanotube segments at an uniform speed toform at least two carbon nanotube films; (b3) overlapping the at leasttwo carbon nanotube films to form a carbon nanotube film structure 16.

In step (b1), quite usefully, the carbon nanotube segments having apredetermined width can be selected by using a wide adhesive tape as thetool to contact the super-aligned array. In step (b2), the pullingdirection is, usefully, substantially perpendicular to the growingdirection of the super-aligned array of carbon nanotubes.

More specifically, during the pulling process, as the initial carbonnanotube segments are drawn out, other carbon nanotube segments are alsodrawn out end to end, due to the van der Waals attractive force betweenends of the adjacent segments. This process of drawing ensures thatcontinuous carbon nanotube film is formed. The carbon nanotubes of thecarbon nanotube film are all substantially parallel to the pullingdirection, and the carbon nanotube film produced in such manner is ableto formed to have a selectable, predetermined width.

In step (b3), after being pulled from the array of carbon nanotubes, thecarbon nanotube films can, usefully, be overlapped with each other toform a carbon nanotube film structure 16. It is noted that because thecarbon nanotubes in the super-aligned array in step (a) have a highpurity and a high specific surface area, the carbon nanotube film isadhesive. As such, adjacent carbon nanotube films are combined by van deWaals attractive force to form a stable carbon nanotube film structure16. The number of carbon nanotube films and the angle between thealigned directions of two adjacent carbon nanotube films may either bearbitrarily set or set according to actual needs/use. Quite usefully, inthe present embodiment, the carbon nanotube film structure 16 includes200 carbon nanotube films, and the angle between the aligned directionsof two adjacent carbon nanotube films can, opportunely, be about 90°.

Quite suitably, an additional step (e) of treating the carbon nanotubefilm structure 16 in the cathode 10 of the lithium battery with anorganic solvent can, advantageously, be further provided after step (b).

In step (e), the carbon nanotube film structure 16 can, beneficially, betreated by either of two methods: dropping an organic solvent from adropper to soak an entire surface of the carbon nanotube film structure16 or immersing the carbon nanotube film structure 16 into a containerhaving an organic solvent therein. The organic solvent is volatilizableand can be selected from the group consisting of ethanol, methanol,acetone, dichloroethane, chloroform, and combinations thereof. Quitesuitably, the organic solvent is ethanol. After being soaked by theorganic solvent, the carbon nanotube segments in the carbon nanotubefilm structure 16 can, at least partially, shrink into carbon nanotubebundles due to the surface tension created by the organic solvent. Dueto the decrease of the specific surface via bundling, the coefficient offriction of the carbon nanotube film structure 16 is reduced, but thecarbon nanotube film structure 16 maintains high mechanical strength andtoughness. As such, the carbon nanotube film structure 16 after thetreating process can be used conveniently. Further, after being treatedwith the organic solvent, due to the shrinking of the carbon nanotubesegments into carbon nanotube bundles, the parallel carbon nanotubebundles in one layer are, relatively, far apart (especially compared tothe initial layout of the carbon nanotube segments) from each other andare oriented cross-wise with the parallel carbon nanotube bundles ofadjacent layers. As such, the carbon nanotube film structure 16 having amicroporous structure can thus be formed (i.e., the micropores aredefined by the spacing/gaps between adjacent bundles).

It is to be understood that the microporous structure is related to thelayered number of the carbon nanotube film structure 16. The greater thenumber of layers that are formed in the carbon nanotube film structure16, the greater the number of bundles in the carbon nanotube filmstructure 16 will be. Accordingly, the spacing between adjacent bundlesand the diameter of the micropores will decrease.

It will be apparent to those having ordinary skill in the field of thepresent invention that the size of the carbon nanotube film structure 16is arbitrarily and depends on the actual needs of utilization (e.g. aminiature lithium battery). A laser beam can be used to cut the carbonnanotube film structure 16 into smaller size in open air.

In step (c), the composite film 14 can, advantageously, be formed by thesubsteps of: (c1) modifying the carbon nanotube film structure 16; (c2)coating the modified carbon nanotube film structure 16 with a polymer;(c3) covering the carbon nanotube film structure 16 with tin salt; (c4)hydrolyzing the tin ion in the tin salt to form the nanoscale tin oxideparticles 18; and (c5) heating the carbon nanotube film structure 16 toeliminate the polymer.

In step (c1), the carbon nanotube film structure 16 is disposed in acontainer with an inorganic acid therein at about 90° C. to 140° C. forabout 4 to 6 hours. The ends of the carbon nanotubes are opened by thecorrosion of the acid. Further, the inner wall and the outer wall of thecarbon nanotubes are modified (i.e. surface modification of the carbonnanotubes). In the present embodiment, the carbon nanotube filmstructure 16 is disposed in a super-high pressure kettle with nitricacid therein at about 120° C. for about 5 hours.

In step (c2), the modified carbon nanotube film structure 16 is immersedin a polymer solution for about 5 to 7 hours. After coated with thepolymer, the carbon nanotube film structure 16 is washed to eliminatethe unstable and excess polymer thereon. Quite suitably, the carbonnanotube film structure 16 is immersed in a solution of polyvinylpyrrolidone (PVP) in ethanol or acetone solvent.

In step (c3), the polymer coated carbon nanotube film structure 16 isdisposed in a container with a tin ion solution therein at about 90° C.to 110° C. for about 2 to 4 hours. Then the container is free cooled toroom temperature. As such, a tin salt is formed on the surface of thepolymer coated carbon nanotube film structure 16. The tin ion solutioncan, beneficially, be a solution of tin inorganic salt in water or tinorganic compound in ethanol or acetone. The concentration of the tin ionin the solution can, opportunely, be in the approximate range from 3mol/L to 10 mol/L. In the present embodiment, the polymer coated carbonnanotube film structure 16 is disposed in a super-high pressure kettlewith a solution of dihydrate tin dichloride (SnCl₂.2H₂O) therein atabout 100° C. for about 3 hours. After free cooled to room temperature,the excess and unstable tin salt on the carbon nanotube film structure16 is eliminated by using acetone.

In step (c4), the carbon nanotube film structure 16 is immersed inwater. The tin ion in the tin salt can be hydrolyzed to form thenanoscale tin oxide particles 18 at room temperature. Further, the tinion can be hydrolyzed rapidly at high temperature or in alkalinecondition. In the present embodiment, quite suitably, the carbonnanotube film structure 16 is immersed in ammonia at 50° C. As such, thenanoscale tin oxide particles 18 are formed and adsorbed to the walls ofcarbon nanotubes of the carbon nanotube film structure 16 or filled intothe spacing/gaps in the carbon nanotube film structure 16.

In step (c5), the polymer coated on the carbon nanotube film structure16 decomposes at high temperature. Quite usefully, the carbon nanotubefilm structure 16 with the polymer coated thereon can be heated at about300° C. to400 ° C. for about 20 to 40 minutes in protective gas (e.g.nitrogen (N₂), or a noble gas). In the present embodiment, the carbonnanotube film structure 16 with the polymer coated thereon is heated atabout 350° C. for about 30 minutes to achieve the composite film 14.

In step (d), the current collector 12 can, beneficially, be a metalsubstrate. Quite suitably, the metal substrate is a copper sheet.

It is to be understood that, in step (d), the current collector 12 inthe anode of the lithium battery is optional. In another embodiment, theanode of the lithium battery may only include the composite film 14. Dueto a plurality of carbon nanotube films being overlapped to form aself-sustained and stable film structure, the composite film 14 can besolely used as the anode in the lithium battery without the currentcollector 12.

It will apparent to those having ordinary skill in the field of thepresent invention that the carbon nanotube film structure 16 can bedirectly disposed on the current collector 12 before step (c). It isnoted that because the carbon nanotubes in the super-aligned array instep (a) has a high purity and a high specific surface area, the carbonnanotube film structure 16 is adhesive. As such, in step (d), the carbonnanotube film structure 16 can be adhered to the current collector 12directly.

It is to be understood that the size of the composite film 14 can bearbitrarily set or set according to actual need (e.g. for use in aminiature lithium battery). A laser beam can be used to cut thecomposite film 14 into smaller size in open air.

Before testing of cycle performance of the anode 10, the lithium batterycan, suitably, be assembled and sealed in an argon-filled glove box. Thecathode active material can, advantageously, includes at least onematerial selected from a group consisting of lithium metal, lithiummanganese oxide (e.g. LiMn₂O₄), lithium iron phosphate (e.g. LiFePO₄),and lithium cobalt oxide (e.g. LiCoO₂). The solvent can, suitably, be amixture of two kinds of solvents. The first solvent can, opportunely, beselected from a group consisting of ethylene carbonate (EC), propylenecarbonate (PC), and combinations thereof. The second solvent can,usefully, be selected from a group consisting of ethylmethyl carbonate(EMC), diethyl carbonate (DEC), diethyl carbonate (DMC), andcombinations thereof. The salt can be selected from lithiumhexafluorophosphate (LiPF₆), lithium bis (oxalate) borate (LiBOB),lithium perchlorate (LiClO₄), lithium terafluoroborate (LiBF₄), andcombinations thereof.

In the present embodiment, the cathode is lithium foil and the anode isthe composite film 14 using a 200-layer carbon nanotube film structure16 disposed on the supporting member 12. A weight of the anode is about50 micrograms. The composite film 14 can, beneficially, be cut to around piece at a diameter of about 10 to 15 nanometers. Quite suitably,the diameter of the composite film 14 is about 13 nanometers. In thecomposite film 14, the tin oxide is 23 micrograms. The electrolyte is 1mol/L Lithium Hexafluorophosphate (LiPF₆) filled in ethylene carbonate(EC) and diethyl carbonate (DEC). The volume ratio of EC and DMC is 1:1.

The anode of the lithium battery has high charge/discharge efficiency,high capacity, and good cycle performance Referring to table 1, thecycle performance of the carbon-nanotube-based anode of lithium batteryin room temperature is shown. The discharge capacity of the first cycleof the lithium battery is above 600 mAh/g. The efficiency of the firstcycle is above 65%. After 30 cycles, the capacity loss is less than 3%.

TABLE 1 Charge Current Discharge Current Cycle Number (mAh) (mAh)Efficiency 1 0 0.0507 0 2 0.0327 0.0335 102.3 3 0.0314 0.0327 104 40.0312 0.0319 102.4 5 0.0312 0.0322 103.3 6 0.0316 0.0328 103.6 7 0.03170.0325 102.3 8 0.0317 0.0328 103.5 9 0.0321 0.0327 101.9 10 0.03160.0326 103.3 11 0.032 0.033 103 12 0.0323 0.033 102.4 13 0.0319 0.0329103.1 14 0.0324 0.0332 102.4 15 0.032 0.0325 101.4 16 0.0317 0.0327103.2 17 0.0317 0.0322 101.4 18 0.0314 0.0323 102.6 19 0.0317 0.0323 10220 0.0318 0.0327 102.8 21 0.0314 0.032 101.8 22 0.0315 0.0321 101.9 230.0312 0.0056 18

Finally, it is to be understood that the above-described embodiments areintended to illustrate rather than limit the disclosure. Variations maybe made to the embodiments without departing from the spirit of thedisclosure as claimed. The above-described embodiments illustrate thescope of the disclosure but do not restrict the scope of the disclosure.

1. An anode of a lithium battery, comprising: a composite film, thecomposite film comprising a carbon nanotube film structure and aplurality of nanoscale tin oxide particles dispersed therein.
 2. Theanode of the lithium battery as claimed in claim 1, wherein the carbonnanotube film structure comprises at least two carbon nanotube filmsoverlapped and crossed with each other.
 3. The anode of the lithiumbattery as claimed in claim 2, wherein each of the at least two carbonnanotube films comprises a plurality of successive carbon nanotubebundles aligned in a same direction, the plurality of successive carbonnanotube bundles are joined end to end by van de Waals attractive forcetherebetween, and each of the plurality of successive carbon nanotubebundles comprises a plurality of carbon nanotubes.
 4. The anode of thelithium battery as claimed in claim 3, wherein the carbon nanotube filmstructure further comprises a plurality of micropores defined by spacingbetween the plurality of successive carbon nanotube bundles, a diameterof each of the plurality of micropores is less than 100 nanometers. 5.The anode of the lithium battery as claim in claim 4, wherein thediameter of each of the plurality of micropores is about 60 nanometers.6. The anode of the lithium battery as claimed in claim 3, wherein anangle between aligned directions of the plurality of carbon nanotubes intwo adjacent carbon nanotube films is about 90°
 7. The anode of thelithium battery as claimed in claim 3, wherein the nanoscale tin oxideparticles are adsorbed on walls of the plurality of carbon nanotubes ofthe carbon nanotube film structure or filled into spacing in the carbonnanotube film structure, a diameter of each of the nanoscale tin oxideparticles is in a range from about 3 nanometers to about 100 nanometers.8. The anode of the lithium battery as claimed in claim 2, wherein awidth of the carbon nanotube film structure is in a range from about 1centimeter to about 10 centimeters, and a thickness of the carbonnanotube film structure is in a range from about 0.01 microns to about100 microns.
 9. The anode of the lithium battery as claimed in claim 8,wherein the width of the carbon nanotube film structure is about 5centimeters, and the thickness of the carbon nanotube film structure isabout 50 microns.
 10. The anode of the lithium battery as claimed inclaim 1, further comprising a current collector with the composite filmdisposed on the current collector, the current collector is a metalsubstrate.
 11. A lithium battery comprising at least a cathode, anelectrolyte, and an anode comprising a composite film, wherein thecomposite film comprises a carbon nanotube film structure and aplurality of nanoscale tin oxide particles dispersed therein.
 12. Thelithium battery as claimed in claim 11, wherein the carbon nanotube filmstructure comprises at least two carbon nanotube films overlapped andcrossed with each other.